The Bridgman method is used to produce articles such as turbine buckets by investment casting. Articles produced using the Bridgman investment casting process are characterized by a directionally solidified single grain structure or directionally solidified columnar grains. Articles with such a directionally solidified grain structures are particularly suited for uses in which the principal stresses experienced by the article are parallel to the directionally solidified grain structure.
Investment casting involves replicating the article that is to be cast using wax patterns. The wax may be injected into a metal or plastic mold having a cavity that is in the shape of the article. The wax patterns also replicate the gating/riser/runner system that is used to feed molten metal to form the article. A ceramic shell is then built up around the wax pattern. The shell may be built up by dipping the wax patterns into a ceramic slurry or ceramic slurries multiple times. The ceramic slurry or slurries include binders/additives to assist in fixing the slurry to the wax patterns. Of course, the first applied ceramic layer or face coat is critical as it will determine the surface finish of the article when cast. Alumina/zircon ceramic particles usually are used to form the face layer to provide an improved surface finish.
The wax pattern is dipped in ceramic layer multiple times, for example, up to twelve layers, the ceramic being allowed to dry before application of additional ceramic slurry. As distance from the face coat increases, larger ceramic particles of different composition, for example silica, may be used in subsequently applied layers.
After the investment or shell build-up has been completed, the assembly is then dewaxed in an autoclave at an elevated temperature to remove the wax from the ceramic shell. Next, the mold is preheated to burn off any binders while the ceramic walls of the mold develop sufficient strength to withstand a casting operation. The shell molds are next inspected for cracks and pieces that may have separated, as such spalled shell pieces can lead to defects in the cast article.
Ceramic wrapping pads may be applied to predetermined locations to improve feeding of the metal. The mold is then preheated, usually 50-150° F. lower than the pouring temperature of the metal or alloy that will be poured to fill the mold. Metal or alloy may then be poured into the mold, which forms the article as the metal or alloy cools. The ceramic mold may then be removed after solidification is complete.
The Bridgman process is used with the investment casting process described above. The preheated ceramic mold is transferred to a mold chamber of a casting machine and secured to a chill plate. After vacuuming, the mold is raised into the hot chamber, which is heated electrically by induction coils, resistance wire or MoSi2 rods. In the Bridgman process, the metal or alloy that will form the article is melted in an inert ceramic crucible, most commonly, a zirconia or alumina crucible. Turbine buckets are formed of, for example, nickel-base (Ni-base) superalloys. These Ni-base superalloys are superheated in the crucibles to temperatures 200-300° F. above the melting temperature of the alloy. When the melt temperature is stabilized, the molten metal is poured into the mold cavity. The mold, filled with molten alloy, and a chill plate, positioned below the hot chamber which supports the mold, is then slowly withdrawn from the hot chamber, which is maintained above the melting point of the metal. As the mold filled with molten metal or alloy is withdrawn into the cold chamber from the hot chamber, heat is withdrawn from the mold through a chill plate, causing directional growth of the metal crystals in the mold initiating at the chill plate, the directional solidification advancing into the molten metal. Heat is also dissipated through radiation from the mold surface so that the molten metal close to the mold freeze sooner than the molten metal inboard of the mold surface at a given mold height above the chill plate. The rate of withdrawal of the mold from the heated furnace chamber depends on the specific features of the article, but is usually a few inches per hour and may be varied in accordance with the geometric structure of the article being solidified. After solidification is complete, the shell and any cores that may have been inserted into the mold may be removed.
The Bridgman method successfully produces directionally solidified and single crystal articles having substantially regular cross sections with thin walls. Regular cross sections includes all cross sections in which heat is withdrawn from the advancing solidification front in a more or less uniform manner, producing articles with single crystal or directionally solidified columnar grains. However, in articles having a significant increase in cross section, the amount of heat that must be removed suddenly increases, which slows the advance of the solidification interface, causing the interface away from the side of the mold to lag even further behind that of the molten interface in proximity to the mold.
The alteration of heat flow attendant to a sudden increase in article cross section may result in undercooling of regions of molten metal in front of the advancing solidification front. The undercooling of the molten metal may nucleate stray grains that grow in a direction that is different from that of the desired directionally solidified grains. These stray grains in general form high angle grain boundaries substantially perpendicular to the desired columnar grains and to the principal stresses experienced by the article when placed into service for its intended use, such as a turbine blade installed in a turbine. The grain boundaries associated with these stray grains have been proven to be the source of crack initiation, adversely affecting the creep/fatigue life and leading to premature failure of a directionally solidified article in environments in which creep/fatigue properties of the article are important.
What is needed is an innovative grain starter design that modifies the advancing solidification front such that the probability of nucleating a stray grain is substantially reduced or eliminated, thereby inhibiting the formation of stray grains and the deleterious high-angle grain boundaries associated with such grains.